Active pixel sensors (APS) are image sensors including integrated circuit containing an array of pixel sensors, each pixel containing a photodetector and an active amplifier. Such an image sensor is typically produced by a complementary metal-oxide-semiconductor (CMOS) process. CMOS APS can be used in web cams, high speed and motion capture cameras, digital radiography, endoscopy cameras, DSLRs, cell phone cameras, and the like. Other advances in image sensor technology have been implemented, such as the use of an intra-pixel charge transfer along with an in-pixel amplifier to achieve true correlated double sampling (CDS) and low temporal noise operation, and on-chip circuits for fixed-pattern noise reduction.
Many traditional CMOS imagers utilize front side illumination (FSI). In such cases, electromagnetic radiation is incident upon the semiconductor surface containing the CMOS devices and circuits. In backside illumination (BSI) CMOS imagers, on the other hand, electromagnetic radiation is incident on the semiconductor surface opposite the CMOS devices and circuits. In general, CMOS sensors are typically manufactured from silicon and can covert visible incident light into a photocurrent and ultimately into a digital image.
More generally, electromagnetic radiation can be present across a broad wavelength range, including visible range wavelengths (approximately 350 nm to 800 nm) and non-visible wavelengths (longer than about 800 nm or shorter than 350 nm). The infrared spectrum is often described as including a near infrared portion of the spectrum including wavelengths of approximately 800 to 1300 nm, a short wave infrared portion of the spectrum including wavelengths of approximately 1300 nm to 3 micrometers, and a mid to long range wave infrared (or thermal infrared) portion of the spectrum including wavelengths greater than about 3 micrometers up to about 20 micrometers. These are generally and collectively referred to herein as infrared portions of the electromagnetic spectrum unless otherwise noted.
Traditional silicon photodetecting imagers have limited light absorption/detection properties, particularly in the infrared range. In other words, such silicon based detectors are mostly transparent to infrared light. While other mostly opaque materials (e.g. InGaAs) can be used to detect infrared electromagnetic radiation having wavelengths greater that about 1000 nm, silicon is still commonly used because it is relatively cheap to manufacture and can be used to detect wavelengths in the visible spectrum (i.e. visible light, 350 nm-800 nm). Traditional silicon materials require substantial path lengths and absorption depths to detect photons having wavelengths longer than approximately 700 nm. While visible light can be absorbed at relatively shallow depths in silicon, absorption of longer wavelengths (e.g. 900 nm) in silicon of a standard wafer depth (e.g. approximately 750 μm) is poor if at all.